Add Window, message_box(), and hello world example

`make examples` will build `examples/hello.exe`.
Run example as `PATH=$PATH:.. ./hello.exe`.

Uses Rust 0.6.
Introduce trait Window/WindowUtil
ll: add RegisterClass/CreateWindowEx/DefWindowProc
Update to Rust 0.7
Use std::os::win32::as_utf16_p()
Makefile: fix clean
ll: add msg loop apis
ll: add some paint-related stuff
ll: add {G,S}etClassLongW

Note that they are only usable on 32-bit platforms.
ll: add GetModuleHandleW
ll: add struct CREATESTRUCT
ll: add CreateProcessW and related structs
WindowUtil: add show() and update()
Extract DialogUtil from WindowUtil
ll: PostMessageW, PostQuitMessage
ll: add gdi32::TextOutW
ll: memo on WNDPROC
Update Hello World example

It uses a lot of low-level stuff for now.
make EmptyWindow empty
Give trait Window its own wnd_proc